1. Technical Field
The present invention relates to stators for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of the Related Art
Conventionally, there are known stators for electric rotating machines which include an annular stator core, a stator coil, and an outer ring.
The stator core is comprised of a plurality of stator core segments that are arranged in the circumferential direction of the stator core to adjoin one another in the circumferential direction. Further, to reduce iron loss of the stator core, each of the stator core segments is formed by laminating a plurality of stator core sheets in the axial direction of the stator core. Moreover, the stator core has a plurality of slots that are formed in the radially inner surface of the stator core so as to be spaced from one another in the circumferential direction of the stator core. The stator coil is mounted on the stator core so as to be received in the slots of the stator core. The outer ring is fitted on the radially outer surfaces of the stator core segments so as to fasten them together.
Moreover, Japanese Patent Application Publication No. 2002-51485 discloses a method of shrink-fitting the outer ring on the radially outer surfaces of the stator core segments. More specifically, according to the method, the inner diameter of the outer ring is set to be less than the outer diameter of the stator core. In the shrink-fitting process, the outer ring is first heated, thereby causing the inner diameter of the outer ring to become greater than the outer diameter of the stator core. Then, the outer ring is fitted on the radially outer surfaces of the stator core segments which together make up the radially outer surface of the stator core. Thereafter, the outer ring is cooled at room temperature until the difference in temperature between the outer ring and the stator core segments becomes zero. As a result, the stator core segments are fixed together by means of stress induced by the difference between the inner diameter of the outer ring and the outer diameter of the stator core.
Furthermore, to minimize the iron loss of the stator core, it is necessary to minimize the thickness of the stator core sheets. However, with decrease in the thickness of the stator core sheets, the strength of the same may be accordingly decreased.
Specifically, referring to FIG. 14, the fastening force of the outer ring 37A is applied radially inward to the stator core sheets 36A, inducing stress in the stator core sheets 36A in the circumferential direction of the stator core. Further, the circumferential stress induced in the stator core sheets 36A may cause the stator core sheets 36A to be deformed in the thickness-wise direction thereof (i.e., in the axial direction of the stator core), thereby making it difficult to maintain the fit between the outer ring 37A and the radially outer surfaces of the stator core segments 32A. More specifically, with the deformation of the stator core sheets 36A in the thickness-wise direction, the outer diameter of the stator core segments 32A will be accordingly decreased. Further, with the decrease in the outer diameter of the stator core segments 32A, the fastening force of the outer ring 37A applied to the stator core segments 32A will be accordingly decreased. Consequently, in the worst cases, the outer ring 37A will be detached from the stator core segments 32A, thereby causing them to be separated from each other.